16 BRITISH BIRDS. 



vernal, which, as the breeding season approaches, is 

 prosecuted with increasing rapidity till the goal is reached, 

 whereas in autumn it is only necessary to make such 

 progress as the advancing season demands. The fall 

 journey is only a part of the round trip to and from the 

 breeding station, the one journey being the complement 

 and necessary consequence of the other, the two together 

 constituting the full cycle of migration. 



As already shown, the sole and all-sufficient cause of 

 migration is the necessity of a congenial environment 

 for the reproduction of the species. This need may be 

 met in the seclusion and isolation of a rocky islet or cliff, 

 or the proper conditions may be afforded by an Arctic 

 tundra. The inception of the movement is the periodic 

 necessity of reproduction, and the journey to the breeding 

 station, be it long or short, is made in obedience to 

 physiologic changes which the bird is powerless to resist 

 or control ; the return journey is obviously a natural 

 and necessary sequence. The return of a bird to its 

 particular ancestral breeding station, and the character 

 of the station selected, are as distinctive of the species 

 as are the colour of its eggs, the character of its nest, 

 the peculiarities of its song and call-notes, or the markings 

 of its plumage. It becomes, therefore, unnecessary to 

 ascribe, except figuratively, the cause of the movement 

 to " strong home love," notwithstanding the fact that the 

 individual bird not only returns to the region of its birth, 

 but will often, as is well substantiated, return year after 

 year for many successive years to absolutely the same 

 nesting site. It is also well-known that during the non- 

 breeding season different local races (subspecies) of a 

 widely distributed species are often found associated 

 during their migratory wanderings, and that when the 

 season of reproduction approaches they will take different 

 migration routes to their respective and often very 

 remotely separated breeding stations. 



It is a well known fact than in many species the old 

 birds migrate first, both in spring and fall, the immature 



